1. Field of the Invention
The present invention relates to a method and an apparatus for determining depleted capacity of a battery of CFx type used in an implantable medical device.
2. Description of the Prior Art
So called CFx (carbon monoFluoride) batteries offer the possibility to use fast microprocessors in implantable medical devices like pacemakers, since this type of battery has the capability of delivering current pulses in the milliampere range required by most suitable microprocessors. Further, there is a growing interest in multi-chamber pacing and also in high rate pacing for arrhytmia suppression and termination which also increases the need of the battery to deliver higher battery current. Future products will require high speed and long range telemetry, which also requires higher battery current.
However, the determination of the state of discharge or remaining capacity of this kind of battery currently causes considerable difficulties, since there is no single electrical quantity which is well correlated to remaining usable battery capacity.
The battery voltage exhibits very long time constants after load changes and as a consequence there is no useful relation between the instantaneous battery voltage and the state of discharge or remaining battery capacity unless the battery load is constant. Measuring the battery impedance is not useful either for this purpose, since it does not provide useful data during the whole discharge period but only in the latter part of the battery lifetime. Thus, known conventional methods of determining the remaining capacity of batteries used in implantable medical devices cannot be used for CFx type of batteries.